1. Field of the Invention
The present invention relates to a liquid crystal display device.
2. Description of the Related Art
In recent years, liquid crystal display devices are widely used as information display devices for portable devices. In order to reduce the size of the portable devices, the liquid crystal display devices need to be reduced in entire thickness.
However, the liquid crystal display device is not self-luminous, and hence a planar lighting unit called “backlight” is often required. It is known that the liquid crystal display device used for portable devices employs a system, as a planar lighting unit, in which a light source is provided on a side surface of a light guide plate for the purpose of reducing the thickness of the backlight. In the planar lighting unit as described above, the light guide plate is provided with an appropriate structure for uniformly scattering light introduced from the side surface onto the front surface thereof. Further, in the planar lighting unit as described above, there must be given a consideration to the avoidance of undesired light from being reflected on a screen.
For example, JP 07-43710 A discloses a liquid crystal display device, in which a light guide plate is provided with a plurality of grooves on a surface thereof, the plurality of grooves each having projections projecting outward on both sides of the aperture of the groove. According to the invention disclosed in JP 07-43710 A, light which has entered the projections goes outside to be diffusely reflected by a reflective member, and then enters the light guide plate again.
Meanwhile, JP 2005-251687 A discloses a liquid crystal display device, in which light sources are provided on a side surface of a light guide plate, and a light blocking member is provided on a flexible printed circuit board so as to prevent a color of the flexible printed circuit board from being mixed with colors on a screen.
In general, a light guide plate is made of a transparent thermoplastic resin, such as polycarbonate or polymethylmethacrylate, and is formed through projection molding. However, when the projection molding method is employed, it has been difficult to obtain a light guide plate with a thickness smaller than a certain degree, for example, the thickness of 1 mm or less, due to the reason that the resin is insufficiently filled into the mold or it is difficult to remove the finished product from the mold. For that reason, the applicant of the present invention is considering adding a surface structure on a surface of a sheet made of a thermoplastic material by transfer with the use of a mold. At this time, an appropriate structure which scatters light is formed on the surface of the sheet made of the thermoplastic material described above. At the same time, a light introducing structure is formed thereon, which guides light introduced from the side surface of the light guide plate inside the light guide plate. After that, a periphery portion of the sheet made of the thermoplastic material is cut out by a method such as punching, to thereby obtain a light guide plate with a desired dimension.
However, in the method described above, the dimension accuracy in the cutout process by a method such as punching is not always high, and the geometric position at the end surface of the light introducing structure may be easily misaligned. Therefore, when the liquid crystal display device is assembled, light from a light source such as a light-emitting diode (LED) cannot be sufficiently introduced into the light introducing structure, and there is a fear that the use efficiency of the light from the light source is decreased.